In the related art, there are known oblique viewing endoscopes for acquiring a field of view in an oblique direction, having a prism provided on the optical axis of an objective optical system (for example, see PTL 1). The prism has a wedge shape having an inclined surface inclined with respect to the optical axis of the objective optical system, and, as a result of the optical axis of the objective optical system on the incident side being deflected by the inclined surface, a field of view located in an oblique direction is observed.
By inserting and removing this wedge-shaped prism into and from the optical axis of a direct-view objective optical system, the observation direction can be switched between direct-view observation and oblique-view observation. When the prism is inserted into the optical axis of the objective optical system, the air-equivalent length on the optical axis of the objective optical system changes, displacing the focus point from the appropriate position.